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What is a Microcode update

Processor microcode is akin to processor firmware. The kernel is able to update the processor's firmware without the need to update it via a BIOS update. From Intel's website:

"The microcode data file contains the latest microcode definitions for all Intel processors. Intel releases microcode updates to correct processor behavior as documented in the respective processor specification updates. While the regular approach to getting this microcode update is via a BIOS upgrade, Intel realizes that this can be an administrative hassle. The Linux Operating System and VMware ESX products have a mechanism to update the microcode after booting. For example, this file will be used by the operating system mechanism if the file is placed in the /etc/firmware directory of the Linux system."

Note: Arch Linux does not use /etc/firmware to process the update.

Updating microcode

Install either intel-ucode OR amd-ucode, depending on your processor vendor, and add microcode to the MODULES array in /etc/rc.conf

Reboot your machine and then execute:

# dmesg | grep microcode

The output of this command should indicate the current version of your processor's microcode and whether any additional update was applied to it.

Note: Microcode updates via software are not persistent. In other words, one needs to apply them at each boot which is why it is placed in the MODULES array.

Note: With udev/systemd-tools 185-1 & kernel 3.4.2 microcode was loaded automatically on my AMD opteron system and i didn't have to add it manually to MODULES array.

Note: If you were a previous user of the microcode_ctl package, remove microcode from the DAEMONS array in /etc/rc.conf. microcode_ctl is no longer in Arch's repositories.

How to tell if a microcode update is needed

The best way to tell is to download and install the appropriate microcode update. First load the microcode module using modprobe.

# modprobe microcode

Then inspect dmesg, if it reports that an update was applied, the microcode in the BIOS of your motherboard predates the one in either intel-ucode or amd-ucode. Users should therefore use the microcode update!

Examples, note that in each case, the BIOS on the motherboard is the latest version from each respective vendor:

It is believed that the date returned corresponds to the date that Intel implemented a microcode update. This date does not correspond to the version of the the microcode database included in the package!